It is well known that optical communication systems are being increasingly oriented towards a fully optical processing of the signal, especially if operation at very high bit rates is required, e.g. bit rates in the order of tens of Gbits/s or higher. Communication systems operating at those bit rates generally employ wavelength-division multiplexing techniques and require tuneable optical filters for demodulating and routing the different channels. An additional requirement is that the components utilised be easily integrated.
Recently, filters based on the use of waveguides with Bragg gratings, and more specifically on the use of waveguides coupled by means of gratings, have been proposed. Those filters are tuneable filters, where gratings reflect the tuning wavelength and transmit the other wavelengths: the wavelength reflected by the grating associated to the first waveguide is coupled into the other waveguide arranged adjacent to the first one and exits through it, whilst the transmitted wavelengths exit through the first waveguide. The paper "Spectral characteristics of coupled-waveguide Bragg-reflection tunable optical filter", by J. P. Weber, IEE Proceedings--J, Vol. 140, No. 5, October 1993, discloses examples of coupled waveguide filters, and in particular filters which make use of waveguides coupled contradirectionally, wherein the reflected radiation exits from the second waveguide in a direction opposite to the incoming radiation. By bringing the grating of the first waveguide to reflection or transmission conditions for a wavelength of interest, it is possible to use the device as a switch and to send the wavelength to one or the other output. Tuning, and thus the choice of output, is obtained with an electrical control, as can be clearly deduced by examining FIG. 3 in the paper.
The presence of electrical contacts however gives rise to parasitic capacitances which limit switching speed, and hence also the bit rate achievable by the system. Typically, electrically controlled devices cannot be used for bit rates exceeding about forty Gbits/s. Given the dramatic growth of systems requiring broad bandwidths, such as broadcast systems, Internet-type communications systems, etc., interest is growing in components that allow obtaining much higher bit rates than those indicated, e.g. rates of a few hundreds of Gbits/s.
The possibility of obtaining contradirectional coupling between two waveguides by means of an optically controlled grating is illustrated by G. I. Stegeman and E. M. Wright in the paper "All-optical waveguide switching", Optical and Quantum Electronics 22, 1990, pp. 95 et seq. The paper, however, describes only the theoretical principle and not the practical embodiment of a device exploiting that principle.